What the planning rules allow, and what they require
Before drawing anything, you look at what the commune allows. The local plan — the plan local d'urbanisme — sets the footprint that may be built on, the heights, the distances to be kept from the boundaries and sometimes the appearance of the façades and the roofs. An extension perfectly drawn but larger than the permitted footprint will never exist: so it is the first thing to check, not the last.
Then come the thresholds. As a general rule, an extension creating more than 5 m² and up to 20 m² of floor area or footprint falls under a prior declaration of works; above that, under planning permission. In urban zones covered by a local plan, that 20 m² threshold is raised to 40 m². And where the total area of the home exceeds 150 m² after the works, private individuals are required to use an architect.
In Nice there is a third filter: within the protected areas and the surroundings of listed monuments, the Architecte des Bâtiments de France gives an opinion that is binding, and the time it takes lengthens the calendar. On the hills, rights of light and the rules on slopes weigh as heavily as the local plan.
We put the file together, we lodge it and we follow it through. The work does not start before the consent has been granted and is beyond challenge: it is the only way not to build something that will have to be taken down.
Opening a load-bearing wall: what actually happens
Joining the extension to the house almost always means opening up. A load-bearing wall is not simply cut through: it is replaced, locally, by a structure that takes up the same load. The order of operations is fixed and it allows no short cut. The method in detail — structural study, propping, beam, load transfer — is on our page masonry and load-bearing wall opening.
First the wall is shored on both sides, the load carried down to a support able to take it — which means looking at what is under the floor as well, not only above it. Then it is opened, the lintel or the beam is set in, packed and grouted in, and the props are not struck until it has set. Days pass between the shoring and the striking of the props, not hours.
Sizing that beam is not a matter of experience alone: it is calculated from the loads actually being carried. Where the work calls for it, a structural engineer is brought in, and the study forms part of the file handed over. It is also what reassures a notary or a buyer, years later.
Grafting so that the junction does not show
Two structures of different ages do not move in the same way. The old one has finished settling, the new one is starting. That is why the junction is treated as a technical point in its own right: foundations suited to what is already there, separated or tied in depending on the case, and the joint detailed so that a crack at the junction does not give the extension away after two winters.
Then comes everything that shows. The roof covering has to meet the existing one with no weak point: valley, flashing, the tiled génoise cornice rebuilt. The renders have to meet without a line. The new openings have to take up the proportions of the old ones, or the elevation reads as two houses stuck together. And on a period building the dressed-stone surround, the colour of the render and the roof tile are chosen on site, in the light of the place, not from a colour chart.
The insulation, the services and the conduit runs go in during this phase, before closing up. A conduit forgotten in a new extension is paid for in chases cut into a wall less than six months old.
The real calendar of an extension
An extension project runs in two very unequal stages. The first is administrative: survey, drawings, the file lodged, the application examined, and the period in which third parties may challenge the consent, which runs from the day it is displayed on site. That time depends neither on you nor on us, and it almost always dominates the calendar.
The second is the work itself, which follows a strict order: groundworks and foundations, structure, roof covering, joinery to make the shell watertight and weathertight, then services, insulation, partitions and finishes. Each stage closes the one before it. A wall is not closed up before everything that has to run through it has been run.
The programme issued with the quotation separates these two stages, and distinguishes what depends on us from what depends on a third party. At signature you know where the waiting is, and which of it can be compressed.
